The present invention relates to DC offset correction, and in particular, to providing DC offset correction in multiple mode or multiple band mobile terminals and the like.
Wireless communication systems support various access technologies, each of which can take numerous forms and be implemented in different frequency bands. For example, the standard analog access technology for U.S. cellular applications is advanced mobile phone system (AMPS), which uses a range of frequencies between 824 megahertz (MHz) and 894 MHz. Digital cellular applications typically use frequency division multiple access (FDMA), time division multiple access (TDMA), or code division multiple access (CDMA) access technologies. FDMA systems typically operate between 824 and 893 MHz. TDMA systems normally operate at either 800 MHz (interim standard (IS)-54) or 1900 MHz (IS-136) frequency bands. TDMA technology is implemented in the global system for mobile communications (GSM) standard in three different frequency bands, depending on geography. For example, GSM operates in the 900 MHz and 1800 MHz bands in Europe and Asia, and in the 1900 MHz band in the United States. In addition to GSM, TDMA is used in PCS (personal communication services)-based systems operating in the 1850 MHz to 1990 MHz bands. CDMA systems typically operate in either the 800 MHz or 1900 MHz frequency bands.
Given the lack of standardization and the varying infrastructure for the above systems, mobile terminals, such as mobile telephones, personal digital assistants, wireless modems, and the like, often need to communicate in different bands and operate in different modes, depending on the type of transmission technology used. In addition to providing analog capability, newer phones are supporting multiple modes and frequency bands.
To support such operation, mobile terminals typically incorporate dedicated low-noise amplifiers (LNAS) and associated filtering for each frequency band supported. Further, the down-conversion circuitry is configured to down-convert the received signals from various frequency bands and using various transmission technologies to an intermediate frequency (IF) or directly to a baseband level for baseband processing. The mixers used in the down-conversion circuitry are driven by local oscillators having varying frequencies depending on the mode of operation.
Unfortunately, the local oscillator energy can leak into the system and cause a DC offset in the down-converted signal. The local oscillator energy may leak into the antenna, which results in a signal that is amplified by the LNA and mixed into the down-converted signal, resulting in a DC offset. The local oscillator energy may also leak into the LNA inputs, causing the same result. Further, the local oscillator energy may leak into the input of the mixer, and ultimately be mixed with itself to create a DC offset. DC offsets may also be caused by mismatched devices that create an imbalance among the differential signals or out-of-band signals from other mobile terminals, base stations, and the like. The DC offset is particularly detrimental in systems wherein the down-converted signals are represented as baseband signals. Even in the absence of leakage signals, the differential signals provided by the down-conversion circuitry should have a common DC level. Typically, DC correction circuitry is used to sample the down-converted differential signals and adjust their DC levels to minimize the impact of any DC offset during baseband processing. For multiple band and multiple mode mobile terminals, switching from one mode or band to another typically affects the DC offsets associated with the differential signals, and requires a DC adjustment of these signals prior to receiving signals in each mode.
During DC offset correction, the antenna must be blocked so that the DC correction circuitry does not falsely lock onto an instantaneous signal level associated with a desired or interfering receive signal. Traditionally, there have been three methods to isolate the antenna from the rest of the system. The first method is to use a transmit/receive switch or duplexer to effectively open the path between the LNA and the antenna, and thus avoid any signals being presented to the LNA inputs. This method has proven unacceptable, in that the switch or the duplexer cannot provide complete isolation, and thus allows signals appearing at the antenna to reach the LNA, which results in an almost random DC level at the outputs of the LNAs. Another alternative is to selectively inject the RF path when more isolation is needed. This approach adds complexity and expense to the application. Yet another method is to turn off the LNA, and thus block signals from reaching the down-conversion circuitry. In this method, where the LNAs are deactivated, the DC offset correction takes place without compensating for LO leakage. Thus, when the LNA is reactivated, the DC offset caused by leakage signals is present. In essence, the latter method corrects for circuit imperfections, but does not address DC offsets induced by leakage signals. Accordingly, there is a need for an improved architecture and process for addressing and correcting DC offset injected into down-converted signals due to signal leakage and the like.
The present invention facilitates improved DC offset correction in a radio frequency receiver, which is capable of receiving signals using any number of communication technologies. In the receiver front end, multiple primary low noise amplifiers (LNAs) are provided and individually selected depending on a communication technology associated with a signal to be received. During reception, signals received at an antenna are coupled via a transmit-receive switch, duplexer, or like switching means, to a receive path leading to the inputs of the primary LNAs. The appropriate primary LNA is activated and the signal is amplified by the primary LNA, down-converted, and then processed at baseband. Prior to receiving the signal, the down-converted signals typically require DC offset correction. The present invention provides a dummy LNA and an associated resistive network. Prior to DC offset correction, the primary LNAs are deactivated and the antenna is decoupled from the receive path leading to the inputs of the primary LNAs. A resistance is selected to provide a load at the input of the dummy LNA, wherein the load emulates the input load resistance seen by the primary LNA, which will be used to receive the incoming signal. Thus, the output of the dummy LNA emulates the DC performance of the primary LNA used to receive the incoming signal to allow accurate DC offset correction.
In one embodiment, the dummy and primary LNAs have differential inputs and differential outputs. Further, a filter, preferably an acoustic wave filter, such as a surface acoustic wave filter, is placed between the switching means and the differential input of each primary LNA. Similarly, the dummy LNA has a differential input and output, and the resistive network provides selectable resistances across the differential input of the dummy LNA. The output of the dummy and primary LNAs may be coupled together and to differential inputs of down-conversion circuitry. In the preferred embodiment, the down-conversion circuitry provides quadrature-based down-conversion. Further, the outputs of the down-conversion circuitry are preferably differential in-phase and quadrature-phase intermediate frequency or very low intermediate frequency signals. DC offset correction circuitry is provided to monitor the respective DC levels of the differential in-phase and differential quadrature-phase signals to adjust the DC levels of the respective signals to a substantially common level prior to baseband processing. Although a differential embodiment is preferred, single ended LNA and filter embodiments also benefit from the present invention.
In operation, control logic, such as a central control system, a serial data interface, part of the baseband processing, or a combination thereof, may be used to control DC offset correction. The control logic functions to effect DC offset correction as follows. Upon determining the need to receive an incoming signal, the control logic will decouple the antenna from the receive path via the switching means, select an appropriate resistance of the resistive network for the input of the dummy LNA, and then activate the dummy LNA.
After allowing the system to settle, the control logic will instruct the DC offset correction circuitry to correct the DC levels of the respective differential in-phase and differential quadrature-phase signals provided by the down-conversion circuitry. Once DC offset correction is complete, the dummy LNA is deactivated and the appropriate primary LNA is activated. Finally, the switching means is instructed to couple the antenna to the receive path so that the incoming signal may be amplified by the appropriate primary LNA, down-converted, and processed at baseband in traditional fashion.
Those skilled in the art will appreciate the scope of the present invention and realize additional aspects thereof after reading the following detailed description of the preferred embodiments in association with the accompanying drawing figures.